The invention particularly concerns the manufacture of paper or board by using on-line calendering. In on-line calendering, the calender is arranged immediately after the paper or board machine or a coating line and the web is taken directly to the calender without any intermediate reeling. Conventionally, machine calenders where the web travels between two hard rolls have been used as on-line calenders. Today, softcalenders are becoming more and more common because of the better surface gloss they achieve. Striving for improved surface gloss and smoothness has further prompted the development of multi-nip calenders suited for on-line calendering. The maximum production speed of the supercalenders used previously has been insufficient, preventing their use in connection with fast production lines.
The purpose of calendering is to increase smoothness and gloss and to improve other properties of the printing surface of paper or board. The improved properties upgrade the quality of the final printed surface. The quality and printability of the printed surface a re among the most important quality factors appreciated by paper users. Similarly, the printability of printing board and the quality of the printed surface thereof are important and, in addition, high stiffness and good bulk are often appreciated. Furthermore, a factor affecting product quality is the evenness of the c ross direction profile of the web, i.e., any variations in web thickness should be as small as possible in the cross-machine direction.
Surface smoothness of the product is achieved by exposing the fibre structure of the product to high pressure and temperature by heating the hard calender rolls and by pressing the rolls against one another such that a high nip pressure is obtained in the nip between the rolls. Due to these forces the fibres forming the web reach their glass transition temperature, and the deformation caused by the nip load is permanent. The gliding of the web surface against the roll surfaces may also give rise to alterations in fibre shape, thus enhancing the smoothing effect.
When multi-nip calendering has been used, the paper has usually been manufactured on a paper machine and thereafter coated, if desired. In both cases the coated or uncoated paper has been reeled onto storage rolls and calendered by means of separate calenders. The paper has been dried to a very low moisture content, typically about 1 to 3% of its total weight. Prior to calendering the paper is sufficiently wetted in order to obtain good calendering results. A suitable moisture content for multi-nip calendering is approximately 6 to 10%. The purpose of drying to a low moisture content is to achieve an even cross direction moisture content profile. The short storage time prior to the calendering step also evens out the moisture content profile. In present on-line calendering processes the web is dried to a high degree of dryness whereafter it is rewetted before calendering, and thus, the process is similar to off-line calendering.
The web can be wetted e.g. by means of the water jet damping device described in U.S. Pat. No. 5,286,348, which achieves an even moisture content profile in the cross-machine direction of the web.
The above-described method which comprises first drying and then rewetting the web is hampered by the time required by the absorption and evening out of the moisture, particularly in the thickness direction of the web and at the surface. If the wetting is performed immediately before calendering, the uneven moisture content profile will affect the final surface properties and the quality grade of the paper may be impaired.
Drying and rewetting increase the energy consumption during the manufacture of the product as well as the space required by the equipment when compared to a process which does dot require overdrying and rewetting prior to the calendering step. An uneven moisture content, e.g. surface moisture or an uneven moisture profile in some web direction leads to changes in the properties of the web, such as gloss or thickness profile because moisture has a strong impact on the workability of the fibres. In the case of an uneven thickness profile, problems will occur in winding, which may even cause cross-direction wrinkles in customer rolls because even tightness is not achieved. The wrinkles will reduce the runnability of the product in further processing e.g. during printing in other further processing machines, thus impairing the quality of the product from the customer's point of view.
Moisture profile affects many factors in the manufacture of paper or board as well as in the final quality of the product. One factor worth noticing is that if fluctuations occur in moisture content, drier parts of the web will start to shrink before the wetter parts, which in turn will lead to stretching of the wetter parts. Uneven stretching will then lead to uneven drying shrink, which in turn leads to thickness variations and variations also in other properties of the product.
In modern machines, the moisture content of the paper or board web to be manufactured is controlled in many ways particularly at the beginning of web formation. The most important target of controlling moisture content profile is good runnability of the machine and the product being manufactured, i.e. maximal production output within a given duration is striven for. This is understandable because moisture content profile and tension profile are highly interdependent. Thus, the best possible moisture content profile has been striven for in such parts of the machine where the effect of dampness profile control on runnability is at its greatest. The dampness profile of the finished base web is then not necessarily homogeneous and it is subject to tension. If the web is stored prior to calendering, the dampness will be evened out and the tensions will be relaxed, and thus, the evenness of the final dampness of the web is of less importance. If, however, on-line calendering is used, the homogeneity of the final dampness has a strong effect on product quality and if the web moisture content is controlled by present methods and principles, the properties of calendered paper or board may even suffer, and the desired improvement in the properties of the final product is not achieved. In multi-nip calenders, it is possible to exercise a relatively strong influence on the thickness profile of the web, but in these calenders a very high nip pressure is applied, wherefore the calendering will usually lead to a significant reduction in thickness and bulk when compared to other calender types. Thus, multi-nip calendering is normally used in the manufacture of products of which a high degree of smoothness and particularly gloss is expected.
One very important feature in the calendering process is that calendering is applied to obtain a slick and smooth surface without losing any more stiffness or bulk than is necessary. As the surface of paper or board is subjected to even very high pressure during calendering depending on calender type, the web is compressed, whereby its thickness is reduced and the web is compacted, in other words its mass per volume is increased, i.e. its bulk is reduced. A reduced thickness and bulk of the web will naturally also result in reduced stiffness. As maximal stiffness and light weight per volume unit is normally required of the product being manufactured, it is difficult to match the different effects of calendering with the properties of the end product.
On the other hand, calendering is used to standardize the thickness profile of the paper, i.e. to remedy thickness defects which may have occurred during web formation. The harder the surface of the rolls used, the easier it is to amend the profile, and thus a machine calender will usually obtain the best profile amendment results, and consequently, this is the most important field of use for this type of calenders. Today, a machine calender is used in many paper machines to finish the thickness profile and surface quality of paper such that they meet the requirements set for the final product. This has been so because there are only limited ways of controlling the cross-direction thickness profile on a paper or board machine, and an acceptable thickness profile cannot be achieved without machine calendering. By means of machine calendering it is possible to raise the surface quality of the product such that it meets end users' demands, but the properties of machine calenders are limited when it comes to improving surface quality, wherefore no remarkable improvement in smoothness or gloss can be obtained by means of a machine calender. As the quality requirements set for printing surfaces are constantly on the increase, other calendering methods must more and more often be used in addition to or instead of machine calendering.
Other types of calenders, such as soft-, long-nip or multi-nip calenders, will obtain a considerably improved surface quality, but they have a much weaker thickness profiling capability than machine calenders, mainly due to the softness of the surfaces on the parts which press the web. It is known that with a reduced tensile stiffness of the calender roll coating, the thickness profiling capability of the calender is impaired but its ability to produce a product with good printing properties is improved. As a machine calender has rolls of cast iron or steel, they may have very hard surfaces, resulting in good thickness standardization. On the other hand, the hard surface will exert stronger pressure on the web at its thicker and denser (harder) parts, wherefore the smoothing effect exerted on the web concerns the thicker parts of the web, and thus, surface properties will vary in different parts of the web.